Introduction
After encountering a number of problems with my Feidaxin FD-150A and discovering a serious lack of documentation on it, I've created this topic to share what I know about it. FD-450A users can also benefit from these instructions and documentation because most of the underlying hardware is identical. Maybe this also works with the FD-160A and FD-460A, which appear to be cosmetic variations of the same radio.

I also want to mention that Feidaxin Customer Support does respond to questions and inquiries, and could still be helpful for general topics about the radio. Their English is not the best, however (it's a smaller Chinese company after all), so do bear that in mind. I will attempt to get some replacement parts from them, I will return here with results soon.

Disassembly
I will preface this by saying that you will need solder, flux, solder wick, and basic soldering skills to separate the PCB from the metal frame. You may also want to replace the solder in the radio with your own, if you have confidence in its quality. I use and recommend 60/40 rosin core lead solder. Try to clean up excess flux with alcohol (I use 91% isopropyl alcohol, maybe denatured alcohol also works?) to prevent corrosion. Most (all?) of these screws in here are size 1 Phillips screws.

Basic disassembly, no soldering required: front housing, keypad, LCD area
To take apart the FD-150A, start by removing the headphone/microphone jack cover, which is held on by one screw. Remove the screw, and lift it up from the bottom with a spudger. Flip the radio over, remove the battery, then the two screws holding the battery latch support to the metal frame. Remove the plastic cover around the positive battery terminal, then remove the screw on the terminal (the terminal itself is attached to the radio's PCB, so do not try to remove it). Unscrew the negative terminal from the frame, then remove the terminal and red rubber support from the radio and set these aside (the terminal's screws are different, so keep them paired to their respective terminal). Remove the two long screws securing the front housing to the frame. Pull the volume knob straight off the radio (it comes off and is keyed to verify orientation). Separate the radio's front housing from the frame with a spudger, starting from the bottom. Once most of the front housing is liberated, pull the frame down to slide the volume control stick through its hole in the front housing, and the front housing will be removed. You can stop disassembling here to fix unresponsive keys or clean dust from under the LCD cover. It's much easier to work under the front housing if you desolder the speaker wires from the PCB, and reattach them after you are done working under the front housing. The wires are fragile, so this becomes necessary if one of them breaks.

Separating the frame from the PCB
Once the front housing is removed, you can continue disassembly if it is necessary. Desolder the speaker from the front housing (it's necessary now). Remove the many (9?) brass-colored screws from the PCB that lead into the frame. You'll see a large solder joint in the top left of the PCB; this is the center pin of the SMA connector on top. Use the solder wick to remove as much solder as you can from this joint, then the PCB should simply lift off of the frame. On the back of the radio, on top of a three-pin device towards the bottom of the radio, there is a thermal pad or something; don't lose this, it's probably important for maintaining sane operating temperatures within the radio. Once the frame is separated from the PCB, the rubber gasket can be removed if you want to remove the SMA connector. Note how it is currently installed, as you'll need to know how to put it back once you're done working with the frame (which I think is made from zinc). You can replace damaged SMA connectors, or convert the radio to proper SMA female, by swapping out this connector with a similarly shaped panel mount connector. Generally, you do not need to remove the gasket or the SMA connector.

ReassemblyFrom full disassembly
Reassembly is straightforward. Replace the rubber gasket in its original orientation and position (the raised rings on the gasket at the top should point outwards, and the tabs slide into slots in the frame). Put the PCB back onto the frame, secure all the screws, THEN solder the antenna connector back in place; this order reduces strain and prevents breakage of the radio. Solder the speaker back on, and continue with basic reassembly instructions.

From basic disassembly
Place the keypad in the front housing. This part is, for me, the most difficult. Making sure speaker wires stay clear of the LCD and microphone, push the PCB+frame assembly all the way up to the top of the front housing, then slowly slide the bottom portion into position. Try to keep the gasket from getting caught in the bottom of the front housing while sliding it in, as this will make it even harder to fully close the radio. You also want to make sure that the gasket is positioned properly at the headphone/microphone jack area. Replace the two long screws securing the front housing to the frame. Place the red rubber support for the negative battery terminal in position (the little nub should be directly underneath the terminal, and the bottom of the stepped side in its little spot), then secure the terminal with its screw. Put the positive terminal screw back in place, and replace the insulating sticker. Put the battery pack support back in place, then secure it in place with the two screws. Use the minimum amount of torque necessary to hold this part firmly in place, as the screw holes are a high strain point. Replace and secure the volume knob and jack cover, then reinstall the battery pack.

Fixing Unresponsive Keys
Over time, you'll notice it is taking more force to press keys on the keypad, especially around the edges. This is due to contamination of the key contacts, and is cleaned rather easily (if corrosion is not occurring). Disassemble the radio to expose the key contacts, and with a cotton swab and rubbing alcohol, clean the keypad area of the PCB and the keypad itself. Once the PCB is shiny again, any oily residues are removed from the keypad, and both pieces are dry, reassemble the radio. The keys should begin to work. If they remain completely unresponsive, this indicates corrosion may have damaged the keypad traces. This is best repaired through careful soldering.

Battery Pack and Power Supply
The battery pack is 1200mAh, confirming the number provided by Feidaxin. Inside are five 4/5AA size NiMH cells in series, providing a nominal 6 volts. A full charge is around 7.4 volts. The batteries are connected to the + and - pins on both the charger side and the radio side, and the 'T' pin is simply a 10kΩ resistor to ground. The battery pack charger is based on a LM317, and will work fine off of a standard 12V 1A power supply (the FD-150A is supplied with a 15V supply, but mine failed after a year and 12V supplies were all I had). There is no Li-Ion battery pack available; you would need to make your own with two lithium cells in series, then regulate output to 6V from the nearly 8V produced. The regulator should also be capable of supplying 2A (batteries at least 2C discharge rate for any sane capacity).

The 'battery eliminator' is what you want to use for mobile or base operation: it converts 13.8V power with a cigarette lighter jack into the 6ish volts needed by the radio. This is based on a LM2576T-ADJ switching regulator, and the LM2576T may have a tendency to fail. All voltage regulation is performed at the cigarette lighter jack side, and those 6ish volts are present in the wires leading up to the virtual battery pack. This virtual battery pack is hollow, but contains the 10kΩ resistor across the 'T' pin and wire stubs for battery connections. To me, it looks like these were once battery packs but were disassembled and had a hole cut in the bottom. Once your battery eliminator fails (it will, trust me), it makes an excellent battery pack enclosure if you decide to experiment with your own designs of battery packs. You can also put a connector on that battery eliminator cord and power the radio directly with 6V 1.5A.

Repairing Audio Power Supply Failure
Surviving several years of use, my FD-150A eventually stopped outputting audio altogether after I plugged in my mono-to-stereo adapter for headphone use. My assumption was that this resulted from the audio amplifier failing, as it failed during an audio event. However, upon disassembly and finding a well-documented Toshiba TA7368F 700mW audio amplifier, I decided to verify its functionality. Audio was making it to the input pins, but the output was dead, with somewhere around 1.2V floating on the output pin. I eventually discovered two weird things about the implementation of the IC: power is supplied directly from the batteries with a 700mW SOT23 PNP transistor controlling current flow, and that the voltage is unregulated from the batteries. This is why it is dangerous to use more than ~7.5V to power the radio! I believe the RF finals are also unregulated due to high current draw.

The particular issue I found is when I tested the Vcc pin of the TA7368F: the voltage was only about 2.1V. I suspected the transistor controlling power flow had failed, a sudden need for current for the audio amplifier taking the transistor over its 700mW limit and damaging it. I could not find a transistor at the time, so I removed the existing transistor and connected the emitter to the conductor. Lo and behold, the radio started to produce audio for the first time in years! There was just one problem with this: the audio amplifier was active when the radio was off, as there was no longer a transistor controlling its power. To resolve this with the NPN transistors I had on hand, I connected the base of a 2N2222 transistor to CPU/Flash-ROM power, conductor to the conductor pad of the original transistor, and emitter directly to the audio amplifier. This restored normal functionality to the radio.

[EDIT] July 2016:
I wasn't satisfied with the repair; since the audio amplifier was connected to the speaker, it remained powered whenever the radio was powered. This produced a 'heartbeat' on the speaker that correlated to the power-save switching of various unessential components, such as the receive circuitry. I instead opted to connect a P-channel 2A MOSFET to the output of the digital transistor that controls the squelch-open LED; the LED has battery voltage available to it at all times, and the transistor connects its cathode to ground when squelch opens. Putting the gate of the FET to the output only powers the amplifier when squelch is open, which has the benefit (or detriment, depending on your style) of muting system beeps, including the 1250ish Hz tone on startup. In theory, and likely in practice, this saves more power.

I am beginning to believe the issue is not, in fact, caused by the failure of this transistor, despite it being (almost?) out of specification for the power consumption needed by the audio amplifier. Instead, failure of the circuitry controlling the power state of the audio amplifier may be at fault. Regardless, the problem is solvable.

Reconditioning the Battery
After a while, you'll notice reduced capacity from the battery pack. I pulled my radio out of storage after three or so years of disuse, so battery life was only about 10 minutes idle at this point. Since the batteries are standard NiMH cells, they can be reconditioned like standard NiMH cells. Let the radio run in receive mode with squelch set to 0 until it cannot power on anymore (it'll play the power-up beep in a loop as playing the beep causes the voltage to drop below the minimum required level), then charge the battery for at least 12 hours (the green LED does not indicate a full charge, but rather high-enough voltage in the cells). Repeat this cycle at least three more times, and the battery pack should have significantly higher capacity than before.

I found that the battery contacts were corroding on my battery pack, and that they were starting to sink into the battery pack housing. To alleviate this, I soldered chips cut from a strip of 1/16" stainless steel to the battery pack terminals (sand the metal surface with 600 grit sandpaper before tinning), restoring full contact with the charger and preventing future corrosion. I'll likely seal the edges from dirt ingress with epoxy, and reinforce the inside of the housing around the contacts with more epoxy (soldering melts the plastic a bit, so it is weakened).